Steam generator organization



2 Sheets-Sheet 1 Filed Sept. l6, 1965 DAVID PALCH/K RONALD a. KNUST INVENTOR.

FIG. I

March 7, 1967 ALCHIK ETAL 3,307,523

STEAM GENERATOR ORGAN I ZATION Filed Sept. 16, 1965 2 Sheets-Sheet 2 0A v10 PALCH/K RONALD B. KNUST INVENTOR.

BY 38K 'which occur at full load operation.

Patented Mar. 7, 1967 STEAM GENERATOR ORGANIZATION David Palchik, Bloomfield, and Ronald B. Knust, Simsbury, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Sept. 16, 1965, Ser. No. 487,850 Claims. (Cl. 122-406) This invention relates to steam generators, and in particular to an apparatus for supporting and protecting steam heating surface from excessive heat during low load operation of the steam generator.

Steam generators must, of course, be designed for all conditions, such as temperature level and transfer rates,

It does not necessarily follow that these conditions occuring at full load are the most critical in all portions of the unit. During startup and during low load operation the redistribution of heat absorption, variations in steam flow, and concomitant film conductance variations can set up new critical points. One method of solving such a problem is to overdesign the critical surfaces at full load so that they may safely handle the low load situations. This approach is often uneconomical and in some cases even impossible.

Another solution to the problem is to impose restrictions on operation of the steam generator at these low loads. Since these restrictions interefere with desired flexibility of the unit insofar as operation is concerned, they are undesirable. Even though steps are taken such as increased design tolerances and operating restrictions, it is advantageous to employ a steam generator design which minimizes the problems at low load.

When a steam generator operates at low loads, the heat distribution pattern changes. Heat which is absorbed by direct radiation from the furnace increases substantially in proportion to the flow passing through the steam generator. Accordingly, heating surfaces which have low flows passing through them, and which are exposed to direct furnace radiation, tend to become overheated during this operating period. This situation is further aggrevated where low pressure steam is passing through these surfaces, such as in a reheater, inasmuch as the internal film conductance is poor and, accordingly, the tube metal tends to run at a higher temperature.

Due to this characteristic of the radiant heat absorbing sections, the temperature increase in the fluid passing through these sections is generally high at low load operations. This results in relatively high temperature unbalance between parallel tubes of the section as well as the substantial temperature difference between the inlet and outlet portions of the sections. The expansions associated with these temperature differences lead to stresses and strains throughout the steam generator which are particularly disadvantageous when they involve support tubes.

In my invention a plurality of platens are located at the furnace outlet. These platens, along with the furnace wall tubing, have high flow rates passing through them by recirculating fluid through these sections. These portions of the steam generator which are in the general area of the furnace and therefore subject to the relatively high radiant heat energy at low loads are therefore adequately protected. The platens are located so as to effectively shield other sections of the steam generator which have lower flows from a major portion of the direct furnace radiation. These platens and horizontal assemblies located above them are supported by tubes, some of which are at the inlet temperature of the platen and some of which are at the outlet temperature. The recirculation at low load maintains a low temperature difference across the platen and consequently a low temperature difference between the various support tubes.

It is an object of this invention to provide high flow rates in all surfaces of the steam generator which receive large amounts of radiant heat energy from the furnace while protecting from direct radiation those lower tem perature sections which have low flow rates.

It is a further object of the invention to provide a support for the platens and for the assemblies located in the gas pass downstream of the platens which is free from extensive expansion difficulties at low loads.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction, and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a side elevation of the steam generator illustrating the general layout and showing in particular the location of the platens in respect to the furnace;

FIG. 2 is a detail of a portion of FIG. 1 which illustrates more clearly the arrangement of the platens and support tubes; and

FIG. 3 is a sectional view taken through FIG. 2 selected to more clearly show the arrangement of the support tubes.

Fuel is passed through burners 2 for burning in the furnace 3. Combustion gases formed by the burning of this fuel pass upwardly from the furnace through flue 4 traversing the heating surface and passing downwardly through flue 5. These gases pass to atmosphere through flue 6 which includes an air heater (not shown).

At full load feedwater is supplied to the vapor generator through the economizer inlet header 10 at a temperature of 500 F. This water is passed through the economizer 11 where it is heated to a temperature of 626 F. and then upwardly through the support tubes 12 to the economizer outlet header 13. This water is then conveyed through the mixing vessel 14 and the downcomer 15 through recirculating pump 17 to the waterwall inlet headers 18.

Vertical tubes 19 line the four walls of the furnace and extend longitudinally from the bottom of the furnace to the roof of the steam generator. Water is passed through these tubes from the inlet headers 13 to the outlet headers 20 and in so doing is heated to a temperature of 761 F.

The water is conveyed from these outlet headers through pipe 22 to the rear pass headers 24 and then upwardly through tubes lining three walls of the rear gas pass to the rear pass outlet header 25. The heat absorption in these walls is only nominal and, accordingly, the fluid is heated to a temperature of 765 F.

From here the fluid is conveyed through pipe 27 to the platen inlet header 28 wherein the majority of the flow passes through the platens 29 to the outlet platen header 30. In so doing the fluid is heated to a temperature of 836 F. The fluid leaving the platen 29 then passes upwardly through the support tubes 32 to header 33.

In parallel with these tubes are a plurality of support tubes 34 which pass upwardly from the inlet header 28 to header 33, these tubes supporting the platens 29.

Fluid is conveyed from the header 33 through a boiler throttle valve 37 and thence to the low temperature steam superheating section 38. The steam is then passed through the high temperature steam superheating sections 39 to the outlet header 40 form which point the steam is conveyed to a turbine (not shown). Steam returns from this turbine to be reheated entering the steam generator at reheat inlet header 42. It passes through the low temperature reheater 43 to low temperature reheat outlet header 44 from which it is conveyed to the high temperature reheat inlet header 45, thereafter passing through the high temperature reheater 46 to the high temperature reheat outlet header 47 from which it passes to the lower pressure sections of the turbine (not shown).

Recirculating line 48 carries no fiow at this time with stop check valve 49 being closed. The interrelation between this recirculating line and the recirculating pump 17 along with the characteristics produced by this design are discussed in U.S. Patent No. 3,135,252 issued to Willburt W. Schroedter.

During low load operation the recirculating line 48 is active conveying a portion of the how passing through outlet header 33 back to mixing vessel 14 where this flow is mixed with the through-flow passing through the economizer and economizer outlet header 13. This mixed flow passes through the downcomer 15 and is effective to increase the flow through the furnace wall tubes 19, the rear pass wall tubes and the platens 29 substantially above that flow passing through the superheater sections 39 and the reheater sections 46. The arrangement of the support tubes 32 and 34 is more clearly seen with reference to FIGS. 2 and 3. Support tubes 34 leave the inlet header 28 and pass parallel to the platens 29 adjacent the lower edge. At a preselected support location two of these tubes are bent outwardly and pass vertically with one on each side of the platen units. The same support tubes continue upwardly with one passing on each side of the reheater 46 and superheater 39. The assemblies, which are in the same plane as the platens, are supported from the support tubes at those points where they intersect. The support tubes then continue upwardly to the roof 52 where they are supported by the roof seals 53, then passing into the outlet header 33.

The platens 29 are located on 2 ft. 2% inch centers. This wide spacing being required to avoid any possibility of slag bridging between platen assemblies when burning fuels with slagging characteristics. The reheater assemblies 46 and superheater assemblies 39 are on 8% inch centers in order to improve the convective heat transfer rate and obtain more surface in a given volume. Accordingly, there are intermediate assemblies which are not in line with the panels which must be supported. This is acomplished by using the support tube 32. which receive their flow from the platen outlet header. These tubes pass parallel to the platens adjacent the upper edge thereof to a preselected location where they turn upwardly. Before they reach the elevation of the lower assemblies, these tubes are bent outwardly from the plane of the platen so that one tube passes up on each side of the assemblies. The assemblies are supported from these tubes as they were from the support tubes 34 and these support tubes are, in turn, supported by the roof seal 53.

In this arrangement all of the tubing associated with the platen assembly is arranged in a horizontal and ascending manner so that the entire section is completely drainable. This greatly facilitates acid washing and general cleaning of the internal surfaces of this section. These sections being subject as they are to high rates of radiant heat absorption would be severely damaged if pockets were formed so that during cleaning sediment settled out. It is therefore particularly important that assemblies in a location such as this be drainable to avoid hot spots during operation due to defective cleaning.

The platen 29 passes through the furnace wall tubing 19 in the front wall. As the unit heats up, there is an expansion between the roof of the gas pass and the location where the platens pass through the wall, which is a function of the temperature of the tubes 19 in the walls of the gas pass. The support tubes 32 and 3% also expand due to the temperature at which they operate. While a reasonable difference in expansion can be absorbed by allowing sufficient length for the tubes to fiex, if there is excessive differential expansion between any of these three portions, undue stresses or strains would occur during operation of the unit.

When the unit is operating as described, at 15% load with recirculation, the average metal temperature of the upper wall tubes 19 is 800 F. The average metal temperature of the support tubes 32 is 860 F. while the average metal temperature of the support tubes 34 is 855 F. Inasmuch as the temperatures of these three portions are sufficiently close, there is litle differential expansion and, therefore, little stress set up. The temperature of the various support tubes is extremely close, which is important since it is difiicult to provide suflicient horizontal run to absorb differential expansion between support tubes. The differential between the support tubes and upper wall tubes does not exceed 60 F. and the differential expansion for this can be easily absorbed in the extended horizontal run available.

If this unit had been operating at this load without recirculation, the differential temperatures would be approximately double those previously described promoting undue stresses or strains during this operating period.

Since the platen 29 absorbs much of its heat by radiation, this platen has what is called a rising temperature characteristic. This means that as load is decreased the heat absorption of the platen relative to the output of the steam generator increases. Accordingly, the temperature difference across the platen at low loads increases and tends to be excessive. This creates stresses, not only due to differential expansion between the upper walls 19 and the support tubes 32 and 34, but also between the inlet and outlet passes of the platen 29. Recirculation at low loads avoids the critical temperature difference at these difiicult operating conditions.

The platen inherently suffers unbalance in heat absorption between parallel tubes. When the temperature difference is high, this unbalance results in very high temperatures in some of the tubes. Recirculation also avoids these high unbalanced temperatures. The recirculating pump floats on the through-flow circuit as described in {1.8. Patent 3,135,252. Accordingly, the recirculation is increased with decreasing load, thereby countering the rising heat absorption characteristic.

The platens so located are well protected with the excess flow and are operative to shield the reheater 46 from a high percentage of the furnace radiation which is relatively high as compared to the low flow through the reheater at reduced loads. During first periods of startup there is no flow at all through the reheater, and the shielding by the platens is of particular benefit.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within t e purview of our invention.

What is claimed is:

1. A vapor generator of the once-through type having a furnace; means for burning fuel within said furance; means for conveyong combustion gases from said furnace; a through-flow circuit comprising a first portion including tubular surface lining the walls of said furnace; a second portion including a plurality of platens located at the outlet of said furnace and being positioned parallel to each other, a third portion including steam heat-ing surface located in said gas conveyong means; said first, second, and third portions arranged to convey fluid serially therethrough; low pressure steam heating surfaces located in said gas conveying means; each of said platens comprising a plurality of horizontal intermes-hed tubes and located intermediate said furnace and said low pressure steam heating surface so as to effectively shield said low steam heating surface from a large portion of the radiant heat emanating from said furnace; means for passing a substantially greater quantity of fluid through said first and second portions than through said third portion; said low pressure steam heating surface including a plurality of assemblies, each comprised of a plurality of tubes intermeshed with each other and in the same plane as corresponding panels, and located at a higher elevation; at pluarlity of support tubes for the platens and assemblies comprising pairs of tubes passing parallel to the platens adjacent the lower edge therefor, said tubes dividing and passing upwardly with one tube on each side of the corresponding platen and assembly; means for supporting the weight of the platen and assemblies from adjacent support tubes; said first portion also including vertical tubular surface lining the walls of said gas conveying means and extending upwardly from the furnace to a support elevation above the elevation of the assemblies; and means for supporting said support tubes at the support elevation.

2. An apparatus as in claim 1 including also a plurality of assemblies of low pressure steam heating surface intermediate those assemblies of low pressure steam heating surface which are in the same planes as the platens; a plurality of intermediate support tubes for said intermediate assemblies receiving fluid egressing from the platens, comprising tubes passing parallel to the platens adjacent the upper edge thereof, each of said intermediate support tubes exending outwardly from the corresponding platen and thence upwardly, passing immediately adjacent to an intermediate assembly; means for supporting the weight of the intermediate assemblies from said intermediate support tubes; and means for supp-orting said intermediate support tubes at the support elevation.

3. An apparatus as in claim 2 wherein said means for passing a substantially greater quantity of fluid through said first and second portions than through said third portion comprises a recirculating system superimposed on said through-flow circuit and effective to recirculate fluid through said first and second portions in supplement to the through-flow.

4. A vapor generator of the oncethrough type having a furnace; means for burning fuel within said furnace; means for conveying combustion gases from said furnace; a through-flow circuit comprising a first portion including tubular surface lining the walls of said furnace; a second portion including a plurality of platens located at the outlet of said furnace and being positioned parallel to each other, a third portion including steam heating surface located in said gas conveying means; said first, second, and third portions arranged to convey fluid serially therethrough; each of said platens comprising a plurality of horizontal intermeshed tubes and located intermediate said furnace and said third portion; means for passing a substantially greater quantity of fluid through said first and second portions than through said third portion; said second section including a plurality of support tubes for the platens comprising tubes passing parallel to the platens adjacent the lower edge thereof, each of said tubes passing upwardly immediately adjacent the corresponding platen; means for supporting the weight of the platens from the adjacent support tubes; said first portion also including vertical tubular surface lining the walls of said gas conveying means and extending upwardly from the furnace to a support elevation above the elevation of the platens; and means for supporting said support tubes at the support elevation.

5. An apparatus as in claim 4 wherein the support tubes for the platens comprises pairs of tubes passing parallel to the platens adjacent the lower edge thereof, with said tubes dividing and passing upwardly with one tube on each side of the corresponding platen.

References Cited by the Examiner UNITED STATES PATENTS 1,942,861 1/1934 Huster 122-1 2,091,231 8/ 1937 Shellenberger 122-235 2,416,462 2/ 1947 Wilcoxson 122235 3,185,136 5/1965 Cozza 122406 KENNETH W. SPRAGUE, Primary Examiner. 

1. A VAPOR GENERATOR OF THE ONCE-THROUGH TYPE HAVING A FURNACE; MEANS FOR BURNING FUEL WITHIN SAID FURNACE; MEANS FOR CONVEYING COMBUSTION GASES FROM SAID FURNANCE; A THROUGH-FLOW CIRCUIT COMPRISING A FIRST PORTION INCLUDING TUBULAR SURFACE LINING THE WALLS OF SAID FURNANCE; A SECOND PORTION INCLUDING A PLURALITY OF PLATENS LOCATED AT THE OUTLET OF SAID FURNANCE AND BEING POSITIONED PARALLEL TO EACH OTHER, A THIRD PORTION INCLUDING STEAM HEATING SURFACE LOCATED IN SAID GAS CONVEYING MEANS; SAID FIRST, SECOND, AND THIRD PORTIONS ARRANGED TO CONVEY FLUID SERIALLY THERETHROUGH; LOW PRESSURE STEAM HEATING SURFACES LOCATED IN SAID GAS CONVEYING MEANS; EACH OF SAID PLATENS COMPRISING A PLURALITY OF HORIZONTAL INTERMESHED TUBES AND LOCATED INTERMEDIATE SAID FURNANCE AND SAID LOW PRESSURE STEAM HEATING SURFACE SO AS TO EFFECTIVELY SHIELD SAID LOW STEAM HEATING SURFACE FROM A LARGE PORTION OF THE RADIANT HEAT EMANATING FROM SAID FURNANCE; MEANS FOR PASSING A SUBSTANTIALLY GREATER QUANTITY OF FLUID THROUGH SAID FIRST AND SECOND PORTIONS THAN THROUGH SAID THIRD PORTION; SAID LOW PRESSURE STEAM HEATING SURFACE INCLUDING A PLURALITY OF ASSEMBLIES, EACH COMPRISED OF A PLURALITY OF TUBES INTERMESHED WITH EACH OTHER AND IN THE SAME PLANE AS CORRESPONDING PANELS, AND LOCATED AT A HIGHER ELEVATION; A PLURALITY OF SUPPORT TUBES FOR THE PLATENS AND ASSEMBLIES COMPRISING PAIRS OF TUBES PASSING PARALLEL TO THE PLATENS ADJACENT THE LOWER EDGE THEREFOR, SAID TUBES DIVIDING AND PASSING UPWARDLY WITH ONE TUBE ON EACH SIDE OF THE CORRESPONDING PLATEN AND ASSEMBLY; MEANS FOR SUPPORTING 